Implement a tab system for service docs view

[MBuchalik]

This PR implements the features described in #127: The service docs view can now not only be explored using the groups tree, but also by Responsibles and by Teams.

If a ServiceDoc has no Responsible (or no Team), then this ServiceDoc will simply not be shown at all. In this case, the user of the application needs to switch to the groups tree view to find the ServiceDoc.

This PR contains the following changes:

• Renaming from "Groups Tree" to "Service Docs Explorer". This resulted in quite a lot of changed files.
• The implementation of the tabs and of the two new exploration options (i.e. the grouping by Responsibles and by Teams).
• Fixes of two typos that I noticed while doing the refactorings.
• Fix of the highligting of the menu items in the left menu: When the user opened the Graph View, the menu item on the left was highlighted. But when the user opened the Service Docs Explorer, the menu item would not get highlighted. This was because the matching was done on the full URL, i.e. we checked for the entire route path instead of just the starting part of it.

Screenshots

[georg-schwarz]

Awesome!

Maybe we can "unowned" services into their own "unowned" item in future PRs.

[MBuchalik]

Maybe we can "unowned" services into their own "unowned" item in future PRs.

Sounds good! 👍